1. Field of the Invention
The present invention relates to a process for producing optical elements, specially a process for producing optical elements having refractive index distributions, which may be used for the optical systems of optical instruments such as cameras, duplication machines, optical communication instruments, optical information processing instruments and the like.
2. Description of the Prior Art
The so-call ion exchange method, as disclosed in documents, e.g. Japanese Patent Publication Nos. 816-824/1972, is known as the most common process for producing optical elements having refractive index distributions.
This ion exchange method for producing optical elements comprises thermal diffusion of a suitable species of ions into a base material such as glass so as to distribute the ions in desired concentrations, thereby providing a refractive index distribution in the base material.
The molecular stuffing method is also known as another process for producing optical elements having refractive index distributions. This method comprises filling the fine pores of a porous glass base material with a suitable dopant to distribute it in desired concentrations, thereby providing a refractive index distribution in the porous glass. Japanese Patent Application Laid-Open No.126207/1976 discloses a process for producing a glass product having a refractive index gradient according to the molecular stuffing method.
The ion exchange method, however, is not practical except that monovalent cations having high rates of thermal diffusion are used, in view of mass-productivity since the ion exchange treatment of the base material is time-consuming because of the low rates of thermal diffusion of ions into the base material.
The molecular stuffing method has the advantage of high rates of dopant diffusion into the porous glass and hence a high rate of forming the refractive index distribution. In other words, shorter times are required for the base material treatment with dopants, as compared with the ion exchange method. On the other hand, this method has drawbacks in that the dopant diffusion into the porous glass is difficult to control accurately and hence precise refractive index distribution in the base material is hardly attainable. Another problem in these two conventional methods is that the refractive index distribution can not be formed freely in the base material, because such distribution is brought into existence by diffusion into the base material of the ion or dopant. Hence it is governed solely by the state of diffusion of the ion or dopant.
Japanese Patent Application Laid-Open No.49,639/1983 has disclosed a process for making a pattern of optical property in a flat base plate. Thereby a pattern of optical property is formed in the direction of a surface of a base material.